Alienation 1: From a worker's product.
Alienation 2: From the act of production
Alienation 3: From a worker's human nature
Alienation 4: From other workers
Wikipedia has a great intro writeup here https://en.wikipedia.org/wiki/Marx%27s_theory_of_alienation
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But, what is a "bullshit job"? It's a job where you cannot see what you do, cannot see its impact on the whole of production, cannot see how others fit in with what you do, and siloed so that you do work in exclusivity of others.
But there it is. Marx wrote about these problems, and there were starting to be these problems. Now, we have computation as its own force that serves to radically separate us from each other's contributions, so that *only* the management and owner class can see how things all go together.
And as to Alienation 4, we see this explicitly with "gig workers", being pitted against each other. There's only so much work, and gig companies figured out they could make workers compete with each other to who will accept the least money for a parcel of work, all the while giving up essential social obligations workers fought in blood to get (workers compensation, unemployment insurance, etc).
People believe they're in bullshit jobs because they have been removed as far as possible from what they and their colleagues do, as a control point by the ownership and managerial class.
In reality, almost all jobs are not bullshit.